1. Field of the Invention
The present invention relates to a control device and method with compensation of coordinate calculating, and more particularly to a control device and method that can exactly calculate coordinate information of a position being touched on a capacitive touch panel according to different impedance values of user who press the touch panel.
2. Description of Related Art
Most people often uses electronic devices equipped with a touch-controlled display in their daily lives, for example an automated teller machine or a copy machine. By slightly touching icons shown on the display, a user can easily operate the desired functions. The touch-controlled operation is provided by a transparent touch panel mounted on the surface of the display. The touch panels can be categorized to resistive type, capacitive type and surface wave type. When putting a finger on the resistive touch panel, a voltage signal occurs for calculating coordinate information of the touch point. For the capacitive touch panel, the coordinate information is obtained based on variations of electrical currents since a user's finger can absorb a minor current when touching the panel. Therefore, the coordinate information of the touching point can be calculated.
With reference to FIG. 3, the capacitive touch panel includes a transparent substrate (80) with flat surfaces, a top conductive layer (81) and a bottom conductive layer (82) respectively formed on the surfaces of the substrate (80), and isolating layers (83)(84) respectively formed on an outer surface of the top conductive layer (81) and the bottom conductive layer (82). Because the top conductive layer (81) and the bottom conductive layer (82) are made of conductive material, a capacitor exists between the two conductive layers (81)(82). Therefore, the AC signal input to the top conductive layer (81) can be conducted to the ground through the capacitor. In other words, even the user does not press the touch panel, the touch panel still has electrical currents flowing to the ground. With reference to FIG. 4, the AC signal is input to terminals a and b of the top conductive layer (81). Before been touched, the touch panel already has currents Ia and Ib flowing from the terminals a and b to the ground through the bottom conductive layer (82). When any user presses the touch panel at point P as shown in FIG. 5, the current variations at all corners of the top conductive layer (81) can be detected because human body absorbs a partial current (Ia1+Ib1) while another partial current (Ias, Ibs) flows to ground through the bottom conductive layer (82).
With reference to FIG. 6, if the voltage of the AC signal input to the touch panel is V=Vosin(wt), the detected current values at terminals a and b are respectively expressed asIA=Ias+Iat=Ias0sin(wt+φ1)+Ias0sin(wt+φ2)IB=Ibs+Ibt=Ibs0sin(wt+φ1)+Ibs0sin(wt+φ2).
At the terminals a and b, a phase difference (φ1, φ2) exist between the currents and the voltages. When any conductive object contacts the touch panel at point P, the voltage potential at the point P immediately has a change. Accordingly, the electric field and current distribution over the top conductive layer (81) also change. Changes of the current values and phases can be detected at each corner of the touch panel.
The current variations at the four corner of the top conductive layer (81) are assumed as ΔI1, ΔI2, ΔI3 and ΔI4 respectively. The total current absorbed by human body can be regarded as a summation of ΔI1, ΔI2, ΔI3 and ΔI4 when the user who touches the panel has a relative large resistance. The coordinate of the touching point is expressed by x and y:
      x    =                            (                                    Δ              ⁢                                                          ⁢                              I                1                                      +                          Δ              ⁢                                                          ⁢                              I                2                                              )                -                  (                                    Δ              ⁢                                                          ⁢                              I                3                                      +                          Δ              ⁢                                                          ⁢                              I                4                                              )                                      ∑                      i            =            1                    4                ⁢                                  ⁢                  Δ          ⁢                                          ⁢                      I            i                                    y    =                            (                                    Δ              ⁢                                                          ⁢                              I                1                                      +                          Δ              ⁢                                                          ⁢                              I                3                                              )                -                  (                                    Δ              ⁢                                                          ⁢                              I                2                                      +                          Δ              ⁢                                                          ⁢                              I                4                                              )                                      ∑                      i            =            1                    4                ⁢                                  ⁢                  Δ          ⁢                                          ⁢                      I            i                              
If the impedance value (ZL) is relative small, a partial current will flow to ground. Thus the total current absorbed by human body can not be regarded as the summation of all current variations. In the situation that the user's impedance value (ZL) is relative small, coordinate calculating errors exist.